(1) Field of the Invention
This invention concerns the art of stereolithography, which is a technique for forming solid shapes from a liquid polymer precursor. More particularly, the invention relates to improvements in such polymer precursors. The liquid polymer precursors of this invention produce green products having good strength at high glass transition temperatures. More particularly, this invention concerns a composition of matter useful in stereolithography that includes at least one vinyl ether, at least one epoxy, at least one acrylate and at least one photoinitiator. In addition, this invention is a stereolithography polymer precursor solution that includes a pyrene sensitizer.
(2) Description of the Art
Stereolithography is described in U.S. Pat. No. 4,575,330 to Hull. The method involves building up a predetermined three-dimensional shape by forming a series of layers of polymerized solids. A cross-section of the solid is traced out on the surface of a bath of polymer precursors under the direction of a computer controlled system which causes the liquid to be exposed to a form of radiation such as an ultraviolet laser. After each polymerized layer is formed, it is lowered in the bath so that a new layer can be formed on top.
Stereolithography solutions are disclosed in WO 89/08021 and U.S. Pat. No. 4,942,001. The references disclose the use of a formulation which combines both acrylates and methacrylates. The polymer precursor solution is comprised of resinous polyacrylates and polymethacrylates dissolved in liquid polyacrylates and polymethacrylates.
U.S. Pat. No. 4,844,144, discloses dispersing a thermoplastic material in a stereolithography liquid polymer precursor so that the solid formed by stereolithography contains a thermoplastic material. Such a solid polymer is said to be useful in the investment casting process since the thermoplastic material prevents the acrylate polymer from expanding when the solid is burned out of a mold. The acrylates and methacrylates used as polymer precursors are closely related to those disclosed in WO 89/08021 discussed above.
It is possible to employ polymer precursors based on vinyl ether compounds which have significant advantages over the acrylate-based formulations used heretofore in stereolithography. Vinyl ethers have been mentioned in publications relating to stereolithography. A vinyl ether urethane was disclosed in Example 6 of French Patent 2,355,794. It was combined with a diacrylate and cured by exposure to an electron beam. There was no suggestion in the French patent that such mixtures are useful in stereolithography, since the compositions apparently preceded the development of that technology.
The use of vinyl ethers in stereolithography was suggested in WO 90/01512, in combination with maleates or fumarates and in the presence of specific types of photoinitiators. In U.S. Pat. No. 4,956,198, coatings intended for use in optical glass fibers were said to have application to stereolithography as well. Such coatings included vinyl ether terminated polyurethane produced from certain diisocyanates reacting with polyols and then capping with monohydroxyl vinyl ethers.
WO 90/03989, addressed the use of vinyl ethers in stereolithography using a transvinylation reaction product to react with a diisocyanate to form urethane oligomers. The transvinylation reaction converted a polyhydric alcohol into products having one or more of the hydroxyl groups in the alcohol converted to a vinyl ether group. The mixture was then reacted with a diisocyanate via the residual hydroxyl groups to produce a vinyl ether urethane.
Vinyl ethers have been combined with epoxy compounds for use as coatings. For example, Crivello reported (Journal of Radiation Curing, Jan. 1983, p. 6-13) UV cure of vinyl ether-epoxy mixtures using cationic photoinitiators. The faster curing of vinyl ethers was advantageous, while the epoxy compounds provided their inherent properties to the film.
An important and still frequently encountered problem in using stereolithography fabricated plastic parts is their capacity to soften and subsequently distort at moderately elevated temperatures. This phenomenon to distort when heated can usually be traced to a moderate or low glass transition temperature (Tg) (most stereolithography parts exhibit a Tg in the range of 50.degree. C. to 85.degree. C.) which, ultimately is a function of the polymer precursors used to fabricate the parts. Another problem with current high accuracy stereolithography resins including resins based on epoxy-polyol/acrylate polymers is their water/humidity sensitivity. Water exposure leads to lower effective modulus of elasticity and lower dimensional accuracy rendering the stereolithography parts unsuitable for high temperature applications.
Further improvement in compositions useful in stereolithography have been sought, particularly with regard to improving the accuracy of the finished parts. The present invention relates to improved stereolithography polymer precursor compositions which provide advantages over those previously disclosed.